The described application relates generally to methods and systems involving removal of sulfur species, including removal of sulfur species from a process stream.
Sulfur species are removed from process streams to reduce pollution and prevent release into the atmosphere. Sulfur removal often occurs by hydrodesulphurization. Hydrodesulphurization generally leads to a reaction product that includes hydrogen sulfide (H2S). H2S is a gas that is flammable, explosive and highly odorous with an odor threshold as low as 5 ppb, and has been designated as injurious to health over a working lifetime exposure level of 5 ppmv (threshold limit value). H2S therefore undergoes additional conversion to more elemental components, which may occur using a number of processes, such as incorporating carriers and/or dyestuff intermediates into the hydrodesulphurization solution. While sulfur is recovered in the solution with such processes, there is also an undesirable accumulation of certain sulfur species, such as thiosulfate and thiosulfite. An example is the Stretford Process, a continuous wash that removes hydrogen sulfide and oxidizes it in an alkaline solution. The solution used is considered regenerative and may be recycled through oxidization with air. The primary component recovered in the Stretford Process is elemental sulfur. Unfortunately, thiosulfate as well as sulfate is also irreversibly recovered. The converted thiosulfate present in the Stretford solution reduces the rate of regeneration and, thus, at least about 20-30% of the process solution must be continuously purged and cannot be recycled. In some processes, thiosulfate accumulation requires the entire system to shut down in order to chemically destroy or dispose of the thiosulfate. Disposal of a spent process solution containing thiosulfate along with recovery of chemicals contained in a purged process solution remain costly.